Power amplifiers that amplify modulated signals and low-noise amplifiers used for receivers of modulated signals are desired to have high-level linearity for suppressing deterioration in transmission characteristics caused by spectrum characteristics and/or signal distortion.
When multivalued amplitude modulation is used in wireless communication, it is desirable to suppress non-linear distortion by linearizing the amplitude characteristic of the power amplifier and to apply techniques for reducing adjacent channel power leakage. In general, amplifiers are desired to maintain constantly high power efficiency, but the efficiency and the linearity of an amplifier are usually contradicting characteristics. To improve power efficiency of an amplifier having low linearity, it is desirable to apply a technique that compensates the distortion caused by the amplifier.
One known distortion compensation method is the pre-distortion method. With the pre-distortion method, a desired signal without any distortion is obtained at the output of an amplifier by adding a characteristic opposite to the distortion characteristic of the amplifier to the signal input to the amplifier.
With distortion compensation according to the pre-distortion method, a transmission signal before distortion compensation and a demodulated feedback signal are compared to calculate and update a distortion compensation coefficient using the difference (power difference) determined by the comparison. The distortion compensation coefficient is stored in a memory with an address of amplitude, electrical power, or a function of amplitude and/or electrical power. By multiplying an updated distortion compensation coefficient with the transmission signal to be transmitted next, the gain of the transmission signal is adjusted to have an inverse characteristic of the distortion characteristic of the power amplifier. The gain-adjusted transmission signal is input to the power amplifier. By repeating the process, the distortion compensation coefficient converges to a final and optimal distortion compensation coefficient, and the distortion of the power amplifier is compensated.
To reduce the number of times calculation of the distortion compensation coefficient is carried out, there is a known technique of adjusting the gain of a transmission signal according to a distortion power obtained from a feedback signal. (For example, refer to Japanese Laid-Open Patent Publication No. 2006-270797.)
Recently, as a wireless communication technology, a technology known as Worldwide Interoperability for Microwave Access (WiMAX) has been attracting attention. WiMAX is a technology developed as a method of constructing a wireless metropolitan area network (MAN), which is a wide area network mutually connecting local area networks (LANs) in metropolitan areas and other specific areas, by wirelessly connecting telecommunications carriers and users' homes in replacement of telephone lines and optical fiber lines. WiMAX may cover an area of a diameter of approximately 50 km with a maximum transmission rate of approximately 70 Mbps with one wireless base station apparatus.
Currently, the Institute of Electrical and Electronic Engineers (IEEE) provides a WiMAX communication standard IEEE Std 802.16-2004 for fixed terminals and a mobile WiMAX communication standard IEEE Std 802.16e-2005.
With WiMAX, the transmission power changes in real time by an increase or a decrease in the number of users, and the transmission power level in each symbol interval fluctuates due to an increase or a decrease in the number of sub-carriers per symbol unit of the transmission signal. Thus, a change frequently occurs in the transmission power, causing the transmission power level to change after a distortion compensation coefficient is calculated at a certain transmission power. As a result, it is difficult to constantly obtain an optimal distortion compensation coefficient. Non-linear distortion due to the transmission power fluctuation occurs even when the rate of convergence by updating the distortion compensation coefficient is increased. Deterioration in the transmission quality due to adjacent channel power leakage is unavoidable until the distortion compensation coefficient converges again.